We are Flooded with Renewable Energy Every Day
The sun drives both solar and wind energy production - but which has the greater potential?
Would it be surprising to hear that the Earth receives from the sun an amount of energy that is about 8, 700 times total daily global demand for all types of energy?
While this is true, there are many steps between sunlight at the top of the atmosphere and electrons traveling along the grid or into your appliances or vehicle. Capturing that energy and making it available everywhere 24/7 is the challenge.
There are two major ways that we currently capture the energy received from the sun: directly for heating or generating electricity (photovoltaic solar panel collectors), and indirectly by tapping the energy in winds created from the sun's energy by the global weather system.
Solar and wind resources are often described together as they are the two most important renewable energy sources, but there are some big differences in both the amount of energy available and the kinds of systems that can be used to tap each of these two resources and distribute the energy captured.
For either wind or solar energy, the major challenge is overcoming the inconstancy of supply. Integrating both into an energy system can help, as it can be windy at night, and the sun can shine on completely calm days, but storing energy when it is available for times when it is not is the biggest limitation. Storing energy is the topic for the next essay where we will explore both conventional and innovative methods.
For this essay, let's talk about how much of that gift from the sun might be captured for our use.
In an earlier essay, that superabundance of solar energy was reduced by the amount reflected and absorbed by the atmosphere, by limiting the placement of collectors to land surfaces that were ice free, and finally by the efficiency with which solar panels convert the sun's energy to electrical energy (see the figure). All values here are in terawatts (TW) which is a trillion Watts of power - the instantaneous flow of energy - but don't worry about the units, it is the relative numbers that are important.
These steps reduced that 8,700 multiplier all the way down to "just" 300 times global energy demand (for all forms of energy, not just electricity). This translates to needing less than 1% of the land surface to capture enough energy to drive the global economy.
Those numbers (300 times and 1%) are so astounding to me that I have to repeat the calculations every time I use them!
We can follow the same steps for wind energy (numbers in the figure are drawn from a paper by Lu and others, 2009).
Of the 173,400 TW of power received from the sun, about 1% is converted to wind through differential heating of the atmosphere, land and water. Restricting energy capture to land and near-shore reduces this potential to 340 TW.
The actual amount of energy in wind that can be converted to electricity at any location on land is related to average wind speed, landform and even land use and vegetation type (this analysis excluded urban and forested areas). Offshore, potential sites are limited by water depth and distance from the coast. The final number in that 2009 paper for global potential energy gain is 96 TW, or about 4.8 times global energy demand. For the U.S. alone, the numbers were 7.8 TW and 2.8 times current annual power demand (again, these are numbers for total power requirements, not just electricity).
The rough calculations in these two figures suggest that the energy is there to power much of the global or U.S. economy with renewable solar and wind sources, although the potential for solar appears far greater (from the figures, 300 times global demand for solar versus 4.8 for wind)!
Technologies for capturing wind and solar power seem to be moving in opposite directions. Growth in wind energy is increasingly focused on larger and larger individual turbine systems, embedded in large wind "farms," especially offshore. The biggest trend in solar is the tremendous drop in the cost of individual photovoltaic solar panels (as much as 80% since 2010).
The installation and operation of wind and solar systems are affected strongly by subsidies and tax considerations such that the actual cost of different approaches can be hard to assess. On the other hand, this also emphasizes that politics and policy (which are not topics for this Substack site) can create powerful incentives or disincentives that shape our energy system (no surprises there). The consensus appears to be that both wind and solar energy systems carry similar costs, and that both are now competitive with fossil fuels.
Perhaps the biggest differences between solar and wind relate to impacts on the landscape and demands on a centralized grid system.
Wind turbines are getting bigger, and spacing between turbines needs to be somewhere between 6 and 12 times the diameter of the rotor (blades). On the biggest land turbines, rotors are up to 164 meters in diameter, requiring as much as 2 square kilometers per turbine. The largest offshore turbines have blades up to 260 meters in diameter.
The actual area occupied by wind farms on land, and the amount of energy captured per unit area, varies widely. Two online sources give a range of 4 to 60 megawatt capacity per square kilometer (a megawatt - MW - is one millionth of a TW, but again, not to worry about the units!). On the other hand, a compilation of energy production and site size for the largest onshore wind farms in the U.S. gives a range of 0.5 to 5 MW generation per kilometer square area. This wide range also points to siting requirements for wind farms. Local conditions and politics can drive the amount of area set aside for these large farms.
An average value of 2.5 MW per square kilometer of actual generation for land-based wind farms translates to a requirement of about 14% of the land area of the lower 48 to meet the 2.78 TW energy demand of the U.S. A similar calculation for solar suggests less than 1%. These numbers are just for comparison, but again this emphasizes that the potential for solar is many times greater than for wind, based primarily on the total amount of energy available.
For wind, large farms or even smaller local-scale turbines tend to have major visual impacts. While some view these turbines as kinetic art, others find them intrusive. This has made siting of wind farms problematic at best and leads to some of the wide variability in power generation per unit area. Some residents living near the large wind farms have filed suits regarding both visual impacts and noise.
Solar systems formed by combinations of small, individual solar panels can be adapted to a wide range of locations and settings and generally have less visual impact. They can also be installed in areas with multiple uses. Solar canopies over parking areas seem to be gaining favor, providing both shade and allowing recharging of electric vehicles underneath. There are also examples of animals grazing under solar collectors, which can provide shade and cooler temperatures.
Building rooftops are perhaps the best example of multiple use for solar collectors. At least two major retail chains in the U.S. (Target and Walmart) are making use of this otherwise wasted space to reduce energy costs and impacts.
A recent study by the U.S. Department of Energy analyzed the potential for rooftop photovoltaic electricity generation using a detailed analysis of roof area and orientation and concluded that small buildings alone (up to 5,000 sq feet) could provide about 25% of total national demand for electricity. Another analysis put total rooftop potential at 40% of U.S. electrical demand. All of these options would reduce the estimate of the percentage of total U.S. land area needed to meet total energy demand to much less than the 1% cited above.
In addition to making use of "wasted" space, these local-use systems can reduce the demand for transporting electricity across the grid, in contrast to the large wind farms that increase demand for grid capacity. One could also argue that a distributed system of small, repeatable solar panels reduces the chance of what has been called single-point failures. Losing one solar panel or even a full roof-top installation is less of a hit to the regional energy system than losing a 14 megawatt turbine, or the infrastructure that connects that turbine to the grid.
Another way to say this is that the infrastructure is already in place for linking small-scale, distributed systems. Net metering (using power from the grid when your local system is not producing - e.g. at night - and returning power to the grid when it is) is a controversial process and a little more on that in the next essay, but it is technically feasible and rapidly becoming a standard practice. How much of this is allowed to happen is primarily a question of politics and policy, and again off-topic for this site.
So the total potential energy available through wind and sun suggests it might be possible to meet a large fraction of our global energy needs by tapping these two renewable sources. Two limitations condition that statement: storing energy for use for calm times in the dark, and driving more of the global energy system with electricity rather than fossil fuels. This second challenge includes building and maintaining a grid system that can handle the additional load.
The world is already headed in the direction of an all- (or mostly-) electric economy, led by forward thinkers that include the likes of Bill Gates (2021). The fairly rapid growth in everything from electric vehicles to electric commercial and domestic power tools may auger rapid additional conversions.
The strain that massive electrification could place on our already troubled electric grid system (discussed in an earlier essay) may be another argument for more localized generation and use of solar power. In effect, net metering (and storage as well) reduces the need for long-distance transport of power across the grid. Local systems can then tap centralized power plants to provide backup to overcome local variability in energy supply from the sun.
While the potential of sun and wind power are expressed here relative to total energy requirements at the global or country level, it is unreasonable to expect either the total electrification of the economy or a complete shift to wind and solar to provide all that power will happen quickly, if at all. In his latest book, How the World Really Works, my favorite contrarian thinker on energy, Vaclav Smil, makes very plain just how deeply conventional materials and sources of energy are embedded in our lifestyle and economy. Change will be slow.
On the other hand, it seems clear that it is possible to speed that transition, and reduce our impact on the global climate system, by acknowledging that the power of sun and wind deserve our primary attention as we modernize or upgrade our energy system.
Sources
The earlier essay on solar power is here:
The 2009 article on wind energy potential is:
Lu, X., M. B. McElroy and J. Kiviluoma. 2009. Global potential for wind-generated electricity. Proc. National Acad. Sci. 106:10933-10938. doi.org/10.1073/pnas.0904101106
https://www.pnas.org/doi/10.1073/pnas.0904101106
for spacing of wind turbines::
60 acres per megawatt needed
https://www.landmarkdividend.com/wind-turbine-lease-rates-2/
4 acres per MW needed
https://sciencing.com/much-land-needed-wind-turbines-12304634.html
Data on energy generation and area required for the largest wind farms in the U.S. are here:
https://en.wikipedia.org/wiki/Wind_power_in_the_United_States
Data on cost of solar photovoltaic systems can be found here:
https://www.nrel.gov/docs/fy22osti/80694.pdf
and potential for rooftop solar here:
https://www.osti.gov/pages/servlets/purl/1462462
25% of national electric demand from small roofs.
This puts total rooftop potential at 40% of electricity
https://arstechnica.com/science/2018/02/a-solar-panel-on-every-roof-in-the-us-here-are-the-numbers/
A story on the use of large retail stores for solar energy gain is:
https://www.nytimes.com/2019/10/07/business/energy-environment/rooftop-solar-panels-retailers.html
Bill Gates' Book is:
Gates, B. 2021. How to Avoid a Climate Disaster. Knopf
Vaclav Smil's book is:
Smil, V. 2022. How the World Really Works. Viking